This invention relates to a system for positioning a transducer on a selected track of a record disc file, and more particularly to improvements which allow more accurate alignment of the transducer with the selected track.
In those types of data storage systems employing rotating discs, such as a magnetic disc file, the discs are rotated at a constant speed and the transducers, capable of either reading or writing, are flown over the disc surfaces on an air film. Addressing stored data is accomplished by means of selecting a transducer, specifying the track position for the transducer, and specifying the segment or sector of the track to be accessed. The transducer positioner is controlled in both the velocity and position modes by a servo-control circuit. There is usually one transducer for each surface of each disc, but all are positioned in unison by the positioner. Once properly positioned to the addressed track, data transfer to or from the track is effected upon selecting the transducer associated with the specific disc surface bearing the track to be accessed.
In the past, it has been common to provide a track density of 100 tracks per inch (TPI) on magnetic disc surfaces using conventional heads for the transducers to either read or write. In systems employing removable discs, the limit on track density has not been the effective recording or reading width of the head but rather the accuracy with which the heads can be aligned with respect to a fixed track in space since a disc might be recorded in one disc memory drive unit and then placed on another for reading, or in another position of the same unit, in the case of a unit having more than one removable disc. If all units are not aligned with respect to a fixed track in space, the positioner could misalign the head to a position between tracks or even on an adjacent track. Other record storage systems employing a rotating disc would suffer the same limitation, such as a storage system employing magneto-optic techniques.
A widely used technique for aligning heads in a magnetic disc file is to place a master disc on the drive unit with a reference track prerecorded with a high degree of precision at the same track address on both sides of the disc. The operator then aligns the heads on both sides of the disc with the reference tracks. Thereafter, upon addressing any track on any disc placed on the unit, the right track will be accessed with a high degree of reliability even though tracks thereon may have been recorded on another unit, so long as the other unit has been aligned using the same master disc, or another master disc having its reference tracks recorded with the same precision. This is so because the positioner servo-control circuit will respond to the track addresses numbered consecutively from 000 for the outer track to position the head through one unit of space for each unit in the number of the track address. Consequently alignment of one head with respect to a reference track on a master disc guarantees alignment of the head for all tracks. That is inherent in a position servo-control circuit.
Recording reference tracks on master discs with sufficient precision is feasible with appropriate laboratory apparatus. Once a master disc has been properly prepared, it can be used repeatedly to align heads as disc memory drive units are installed in the field, thus justifying the expense of preparing a master disc.
While master discs have been widely used for alignment of heads in disc files having a track density of 100 TPI, such master discs have not heretofore been used for alignment of heads in systems having a greater track density, not because the master discs cannot be produced with higher precision, but because it is too difficult for the persons who make the field alignment to adjust the heads to a reference track with the higher degree of precision required for a density greater than 100 TPI. Usually the operator must make a head alignment by adjustment of assorted mechanical techniques, such as set screws on opposed cam surfaces that cause the arm or lever which carries the transducer to slide along a predetermined path. In a typical instance, two offset screws are employed. One offset screw must be turned counterclockwise to slide the transducer arm in one direction while the other is turned clockwise. The extent of movement achieved depends upon a delicate balance between how much one offset screw is turned counterclockwise while the other is turned clockwise.